The present invention relates to a method and an apparatus for removing carbon dioxide, useful for life support systems and cabin circulation systems.
The continuous removal of metabolically produced carbon dioxide in life support systems and cabin circulation systems is necessary in order to make quality breathable air available as required by humans. Because of physiological requirements, the carbon dioxide content of breathable air must not exceeded 0.5%. Residual oxygen content at such carbon dioxide concentrations is adequate for the respiration function; however, prolonged exposure to higher carbon dioxide concentrations leads to physiological disorders with, in some cases, a dramatic decrease in reactivity. Currently, the biochemical mode of action is largely not understood and cannot be explained solely by an oxygen deficiency.
Typical areas of application, where removal of carbon dioxide from the breathing air is essential, are primary systems with completely closed air circulation, such as in manned space flight systems and submarines used for military or civilian purposes. Furthermore, applications in the areas of civilian air travel and high-speed railroad systems (conventional and linear motor systems) can be identified, which envision cabin air circulation that is sealed to the extent of about 85%, in order to save energy for heating, cooling and preparing air.
In each case, such sealed systems require continuous removal of metabolically produced carbon dioxide.
Currently, completely closed circulation systems such as in the MIR space station and submarines, remove carbon dioxide with solid absorbers. An example of a solid absorber is the use of lithium hydroxide cartridges, in which the carbon dioxide is removed by the irreversible formation of lithium carbonate from the hydroxide. After the loading limit has been reached, the absorber must be exchanged. At the present time, lithium hydroxide systems cannot be regenerated.
Regenerating systems based on materials which selectively absorb carbon dioxide are known in the area of chemical process technology and are state of the art, for example pressure change adsorption, with zeolite absorbing materials as well as liquid phase absorption using organic nitrogen bases, such as ethanolamine, with thermal or pressure-change regeneration. The typical area of application of such processes is in large stationary installations. These technically established processes can not easily be transferred to mobile systems. Additionally, by their very nature, such systems are discontinuous processes and require extensive chemical engineering effort in order to develop a quasi-continuous process. Continuous gas removal processes which can be operated with justifiable process technology effort, require the use of membrane-supported methods.
It is necessary to distinguish between membrane-supported absorption/desorption (for example, as disclosed in TerMeulen et al., WO 94/01204) and a true gas permeation membrane separation process. The present invention relates to a true gas permeation membrane separation.
Classical membranes used in gas permeation are solution-diffusion membranes, with a dense, active separating layer of polymer materials. Materials and performance data disclosing permeability and selectivity of such membranes for the separation of carbon dioxide from air are disclosed in Jehle et al., "Concentration and Subsequent Methanation of Carbon Dioxide for Space and Environmental Applications", Carbon Dioxide Chemistry, Env. Issues, The Royal Soc. of Chem., Stockholm 1994, pages 261-269. As can be seen from this disclosure, the presently known polymer membranes cannot be used for mobile applications, which are of primary interest here, because of the anticipated volume and weight of the module.
An improvement, specifically in carbon dioxide transport properties, for gas permeation processes is possible through the use of liquid membrane systems with so-called "carrier effects". Such carrier liquid membranes are characterized by a substance which preferentially binds the material to be transported in a selective and reversible manner. This substance is added to a solvent, which for membrane systems is often water. As such, it is possible to increase the transport rate by several orders of magnitude in comparison to the material transport rate naturally existing for a particular solvent. The latter is determined by the physical solubility of a substance in a solvent as well as by the corresponding diffusion coefficient.
FIG. 1 diagrammatically shows the basic concept of a carrier-borne transport process in a liquid membrane. The essential distinguishing feature is the fact that the carrier molecule, as well as the carrier/carbon dioxide complex, are free to move in the solvent. However, directed carbon dioxide transport through the liquid membrane can only occur if the carrier/carbon dioxide complex can dissociate into the starting components on the permeate side of the boundary phase. This process is thermodynamically controlled according to the following equilibrium: ##STR1##
K.sub.c is the reaction constant for the process. The equilibrium can be controlled exclusively by the partial pressure of the carbon dioxide. In each case, a resulting carbon dioxide flow through the membrane by carrier transport requires that the partial pressure of the carbon dioxide be lower in the gas space on the permeate side than in the gas space on the feed side. Therefore, high carbon dioxide concentrations favor formation of the carrier/carbon dioxide complex and low carbon dioxide concentrations favor the decomposition of the carrier/carbon dioxide complex.
The following measures are important for continuous gas permeation membrane processes:
i) Vacuum operation on the permeate side. The vacuum, produced by the vacuum pump, must be adapted so that at all times, the partial pressure of the carbon dioxide is lower on the permeate side than on the feed side. PA1 ii) Flushing gas operation on the permeate side. The pressure level of the flushing gas is not important. Only the ratio of the volume flow of flushing gas to the volume flow of permeate gas under steady state operation is important. For the transport of carbon dioxide through the membrane, it is necessary that, as a result of the dilution effect of the flushing gas with respect to the carbon dioxide, the partial pressure of the carbon dioxide on the permeate side is less than the partial pressure of the carbon dioxide on the feed side.
It will be apparent that measures i) and ii) can also be used in combination (with no pressure level limitation for ii)).
FIG. 1 furthermore shows the basic, necessary arrangement of a liquid membrane, in this case, a liquid membrane based on water. The membrane liquid is between microporous, hydrophobic membranes, which usually consist of polymer materials, such as polytetrafluoroethylene (PTFE), polypropylene or polyvinylidenefluoride (PVDF).
At first, the gas exchange takes place without interference; liquid is prevented from emerging by the hydrophobic nature of the materials. If the liquid membrane is at rest, the system involves an immobilized liquid membrane. Methods for removing carbon dioxide with such membranes are disclosed in U.S. Pat. No. 4,750,918. Such methods involve continuously removing metabolically produced carbon dioxide from air, such as the air found in life support systems and cabin circulation systems, wherein the air containing metabolically produced carbon dioxide is passed through a first set of hollow fibers, which are embedded in a carbon dioxide-selective liquid membrane and the carbon dioxide-rich permeate is drawn off through a second set of hollow fibers, which are also disposed in the membrane. Solutions, consisting of potassium carbonate/potassium hydrogen carbonate with organic nitrogen bases such as monoethanolamine (MEA) and diethanolamine (DEA), are taught as selective carriers. The selective and reversible nature of the carbon dioxide bonding by organic nitrogen bases is established from gas-scrubbing methods (such as the activated MDEA method of BASF AG in Ludwigshafen, Germany).
A different possibility for constructing immobilized liquid membranes is to impregnate porous carrier structures (membranes) with an active removing liquid as disclosed in EP 0 309 259 B1.
The immobilized liquid membrane variations are clearly limited with respect to their commercial use for the removal of carbon dioxide with water-supported liquid membranes.
A problem not solved by such systems is that a constant loss of solvent (water) is noted due to the vacuum and/or flushing gas operation required on the permeate side of the system.
While moderate water losses from the liquid membrane do not interfere with the carrier transport mechanism, the water loss is much more serious with respect to the ability of the hydrophobic membranes to retain the removing liquid. Such processes involving membrane systems do not exhibit long-term stability unless water is supplied continuously.
A further disadvantage of such systems with immobilized liquid membranes is the fact that the separation capacity required for use in life support systems or cabin circulation systems leads to a relatively large exchange surface and thus to a large overall size of the separation system.
In addition to immobilized liquid membrane systems, flowing liquid membrane systems based on plate and frame modules for removing carbon dioxide are disclosed, for example in, Teramoto et al., Kakakukogaku Rombunshu 16, 6 (1990), Eido T., et al. Japanese Published Application No. HEI 2-246989 (1990), and Jehle, W., et al., SAE Technical Paper 941339 (1994). Teramoto and Jehle use experimental laboratory equipment to determine the relevant transport resistances. Solutions of potassium carbonate/potassium hydrogen carbonate with organic nitrogen bases (MEA, DEA) are used as selective membranes.
In the case of such plate and frame modules, there are significant problems with sealing the individual plates. This problem has yet to be solved for units with a larger number of plates, as required to be economically useful.
It is an object of the present invention to provide a method with high operating efficiency and good separating power for the removal of carbon dioxide from air in closed systems such as in life support and cabin circulation systems. This objective is accomplished by a method for continuously removing metabolically produced carbon dioxide from respired air, such as the air found in life support systems and cabin circulation systems, in which the carbon dioxide-containing respired air is passed through a first set of hollow fibers, which are embedded in a carbon dioxide-selective liquid membrane and the carbon dioxide-rich permeate is drawn off through a second set of hollow fibers, which are also disposed in the membrane, and in which the liquid membrane is circulated.
According to the present invention, two sets of hollow fibers are disposed in a circulating liquid membrane, which is selective for carbon dioxide. Carbon dioxide-containing respired air flows through a first set of hollow fibers and the carbon dioxide-rich permeate stream is discharged through a second set of hollow fibers.
Improved removal is achieved by virtue of the fact that a flowing liquid membrane is used. The carbon dioxide transport properties can be influenced further by varying the liquid flow conditions. Additionally, leakage problems, such as those known with the use of plate and frame modules, do not exist.
In a preferred embodiment, in order to facilitate the transport of material, each hollow fiber from the first set can be disposed adjacent to a hollow fiber from the second set.
In a preferred embodiment, a liquid membrane is used, which is based on aqueous, organic nitrogen bases. It may furthermore contain additions of potassium carbonate and/or potassium hydrogen carbonate. In another preferred embodiment, the liquid membrane is an aqueous solution of diethanolamine (DEA) at a concentration ranging from 0.1 to 2 moles/liter.
The concentration ratio of the individual components of the liquid membrane is preferably kept constant. In order to achieve this, individual losses are determined by a measurement and control mechanism, and appropriate amounts are added continuously.
A vacuum operation, a flushing gas operation or a combination of the two can be provided on the permeate side.
In a preferred embodiment, both sets of hollow fibers consist of identical, microporous, polymer material of a hydrophobic nature, for example, polytetrafluoroethylene, polyethylene or polypropylene.
According to the present invention, a technically usable method which exhibits long-term stability and high selectivity is created.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.